|Ask the Experts|
January 17, 2020
Conformal Coating in Low Humidity Environments
Whereas conformal coating is excellent for high humidity protection - is there any performance impact when subjected to very low humidity environments?
|Expert Panel Responses|
Dr. Pauls, could you expand on your answer slightly? We very much respect your knowledge on the subject of conformal coatings, and we are wondering which coating would provide the best protection from Diet Mountain Dew?
I will give you my favorite engineering answer - It Depends.
What do you mean by"low humidity"? And what conformal coating are we talking about? Different chemistries respond to the environment differently.
In general, in low humidity environments, there is no humidity challenge to the coating. Any humidity that may have passed through the coating during humid exposures would diffuse out during very dry periods. The only impact I can see if if you are talking about dessicated storage, like in pure nitrogen or 0-1% RH environments.
Conformal coatings are all organic materials and can dry out with time, though I have not ever seen this. Potentially, coating could crack under such situations as the plasticizers leave the material(assuming they are there in the first place). You would be best tocontact the vendor of the coating you are interested and pose the question. Coating vendors test their products in a lot of weird environments.
Principal Materials and Process Engineer
Not if it is fully cured.
SME - PWB Technologies
Conformal coatings provide a breathable membrane protecting from humidity and contamination. They will all vary slightly in resistivity depending on the exposure time to varying humidity levels.
The optimum properties of a conformal coating are measured when it is fully cured and dry therefore there is no performance impact when they are subjected to very low humidity environments. This is possibly the environment when they are protecting from contamination such as dust!
Some polyurethane and silicone coatings rely on moisture in the air to fully cross link the polymer(fully cure).Curing times could be affected if applied in a very dry environment.
As far as I can tell, there is no downside to using conformal coating in a low humidity environment.
I believe the coating would also act as a barrier to prevent particulate matter from attaching itself to the laminate material and it will also act as a mechanical barrier to protect the surfaces of the board and the components.
Vice President, Technical Director
Low humidity environments have no impact on the performance of a cured conformal coating.However, in some cases the relative humidity level may impact the cure process itself whereby impacting the post cure performance. To understand this it is important to understand how conformal coatings work.
To begin it is important to note with every polymeric material water absorption will inevitably occur. The design of conformal coatings is unique in that the material is able to maintain adhesion to the respective surfaces coated while it absorbs moisture and ultimately reaches equilibrium. With this in mind, the performance of a conformal coating is only as good as the strength to the surface after cure.
Some conformal coatings utilize UV light to polymerize the reaction. Depending on the configuration of the parts, the material may not see sufficient direct exposure from the source to fully drive the reaction. These are called shadow areas, requiring a secondary cure mechanism to complete the reaction.
Many conformal coatings use heat or oddly enough moisture to drive the secondary cure. Wants sufficient moisture is absorbed by the unreacted parts of the polymer and the material fully sets. At this point moisture penetration is defined by the saturation characteristics of the material and the strength of the conformal coating to the surface. If the relative humidity is low in the surrounding environment during cure is low than there is a chance the material may not reach its optimal adhesion whereby affecting performance.
With a fully cured conformal coating the relative performance in a moisture rich (i.e. wet) versus moisture starved (i.e. dry) environment is negligible. The operational life time achieved in a moisture rich environment, such as south east Asia,would be matched or exceeded in a dryer environment, such as the western United States. The main item to consider is how well was the material cured...
The only two downsides of coating in low-humidity environments that I can think of are also present in any humidity level, cost of application and a change in dielectric properties of the assembly. Cost cannot be used as a factor if there is any possibility the CCA will in fact see high humidity or even liquid moisture.
The dielectric properties are usually minor. So the only other possibility I can think of is possible cracking/peeling of the coating under extremely dry conditions, but for most coatings that is not very likely.
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